- Exceptions in PEAR packages need to extend from PEAR_Exception, not Exception, per the coding standards. That should just be a matter of changing the extends clause in Services_SharedBook_Exception from extending Exception.

- You can access SimpleXML attributes more simply than you are as array elements: for example, $apiAnswer->attributes()->status could be replaced with $apiAnswer['status'].

- At present, your tarball isn't installable. If you change the package name to Services_SharedBook in package.xml, it'll work.

Beyond that, I can't test it as I'm not a member of SharedBook at present, but it looks solid. Just a shame about the HTTP_Client situation at present.